Project Summary: Innate immune myeloid cells including monocytes and macrophages are classically activated to clear pathogens and promote immunity, but these same cells are reprogramed within tumor microenvironments to become tumor-associated macrophages (TAMs) where they suppress anti-tumor immunity and promote tumor growth. Studies investigating biological mechanisms behind anti-tumor ?M1-like? classical maturation versus tumor-promoting ?M2-like? alternative maturation have therapeutic potential. Our lab and others have shown that paired Ig-like receptor B (PIR-B), a receptor expressed on myeloid lineages in mice, and suppresses immune activation. We further showed that PIR-B maintains the M2-like phenotype of tumor infiltrating monocytes. Mice deficient in PIR-B have reduced tumor burdens and an infiltrating monocyte profile that resembled M1-like classical activation. The leukocyte immunoglobulin-like receptor subfamily B (LILRB) represents the human ortholog of mouse PIR-B. Like PIR-B, LILRB expression is restricted to myeloid cell populations. Our group generated a panel of antibodies against LILRB family members and screened for functional activity in human monocyte-derived macrophage cell-based assays. Consistent with our findings in mouse, we observed that LILRB antibody antagonists promote classical M1-like activation. Concordantly, macrophages downregulate M2-associated markers and enhance TNF? while suppressing IL-10 secretion, a profile consistent with M1 classical maturation. We hypothesize that LILRB is an important homeostatic regulator that suppresses human monocyte classical activation. Two specific aims will be pursued: 1) To elucidate how LILRB signaling suppresses classical activation in human macrophages. 2) To determine the effect of LILRB blockade on monocyte/macrophage maturation using in vivo patient-derived xenograft (PDX) models. Studies of the cellular and molecular mechanisms of action utilized by LILRB antagonists coupled with in vivo PDX models are critical for understanding LILRB function in myelopoeisis. Successful completion of these studies will result in a better understanding of how findings from mouse PIRB translate to the human LILRB. Furthermore, our PDX models will indicate if LILRB blockade alters the tumor microenvironment and enhances anti-tumor immunity. These findings may lead to the discovery of novel and specific targets on TAMs that can be used to combat the immune suppression associated with advanced malignancies. The ablation of immune suppression should significantly augment the efficacy of existing immune-based therapies for treatment of advanced metastatic cancer.